1. Field of the Invention
The present invention relates to an artificial hip joint adapted to replace and restore the hip joint of a human being.
2. Prior Art
Typically, an artificial joint, and more particularly an artificial hip joint, comprises a stem head ball (hereinafter referred to as a "stem head") integrally fixed to a stem or attached by tapered engagement of the stem with the stem head by the use of metal, such as stainless steel, cobalt chromium based alloy or titanium alloy, in such a manner that the stem is fixedly inserted into a femur by the use of cement in the gap between them. On the other hand, a socket for receiving the stem head therein is made of polyethylene and fixedly mounted on the pelvis with cement.
In recent years, it has become general practice to fixedly mount the stem head on the pointed end of a metal stem by tapered cone engagement, and the kind of the material used in the construction is mostly stainless steel, cobalt chromium based alloy and the like.
The artificial joint constructed of the material described is adapted to vary the height (neck-length) of the joint when the stem is fixedly mounted on the stem head by changing the depth and the diameter of the tapered hole in the head. This joint system is becoming the common practice for the artificial hip joint.
On the other hand, alumina ceramics, low in friction and abrasion by a combination of the material with polyethylene, is recognized as a material for the stem head because of its excellence in clinical performance and has come to be used in many cases. However, a major concern in the past has been the possibility of damage to the stem head due to unfitness in engagement of the pointed end portion of the metallic stem with the tapered hole formed in a ceramic stem head when the ceramic stem head is used. It has been said that a load of up to 5 times the body weight of a human being acts upon the condyle of the human being, and accordingly, a load of up to about 400 kg repeatedly acts upon a person having a body weight of 80 kg. That is to say, the head is always subjected to large cyclic forces for a long time, so that an exceedingly high strength is required. Also from a view point of durability over a long period of time, a high margin of safety is demanded of the hip joint. However, the reality is that, when the proximal end of the stem is fitted into the tapered hole of the ceramic stem head, problems can occur in that, for example, even the misfit caused by fine foreign debris, such an fine bone pieces, straying into the gap between the hole and the end of the stem would lead to an uneven distribution of stress within the tapered hole of the stem head and induce fracture of the stem head due to the local concentration of stress.
A means to solve the problems above described makes it necessary to bring the tapered hole formed in the stem head into complete tapered engagement with the tapered cone formed in the pointed end portion of the stem.
It is, however, impossible to bring both into highly precise agreement with each other by conventional machining.
And, there have been made various proposals such as a proposal for circumferentially forming regularly-arranged concavities and convexities on the surface of the tapered cone of the metal stem (for example, U.S. Pat. No. 4,012,795), or a proposal for making the tapered cone of the stem hollow or forming slits in the cone and deforming the tapered cone of the stem so as to bring the tapered cone of the stem into agreement with the wall surface of the tapered hole formed in the stem head in shape.
However it has still been impossible to prevent the development of cracks in the ceramic stem head and the reduction in bonding strength even by the above described means.
In addition, the ceramic stem head has a hardness remarkably higher than that of the metallic stem head and once the stem head is hammered onto the tapered cone of the pointed end portion of the metallic stem, the inner wall surface of the tapered hole of the stem head scrapes the metal cone and metallic debris is stuck to the inner wall surface of the tapered hole of the stem head to form black stains, whereby a large number of scratches and other irregularities are formed on the tapered cone at the pointed end of the metallic stem, and thus the engagement of the stem head with the tapered cone at the pointed end of the metallic stem is not fully realized even though the doctor has selected the stem and tapered cone, from among the many available, most suitable for the body structure of the patient. That it so say, the doctor has less chance to adequately select the most appropriate stem and tapered cone.
In addition, as proposed in U.S. patent application Ser. No. 07/370,450 in which the present inventor is one of co-inventors, it is desired in order to solve the above described problems that the angle of the tapered cone of the stem is set slightly smaller than that of the tapered hole formed in the ceramic stem head and the tapered cone at the pointed end of the stem is engaged with the depth or the wall surface of the tapered hole formed in the stem to prevent the stem head from being cracked and thereby increase the bonding strength. However, the problem remains that the metallic debris are ground onto the wall surface of the tapered hole of the ceramic stem head and the scratches abrasions and other irregularities can not be prevented from being generated on the tapered cone at the pointed end portion of the stem.